feat: auto-persist metadata and chapter list on first book preview visit

When a book is not in the local DB and the preview endpoint is hit,
metadata and the chapter index are now saved to PocketBase in the
background. Subsequent visits load from the local store instead of
scraping live. Chapter text is not fetched until an explicit scrape job
is triggered.

scraper/internal/orchestrator/orchestrator_test.go

@@ -90,6 +90,10 @@ func (s *mockStore) WriteChapter(_ context.Context, slug string, ch scraper.Chap
 	return nil
 }
 
+func (s *mockStore) WriteChapterRefs(_ context.Context, _ string, _ []scraper.ChapterRef) error {
+	return nil
+}
+
 func (s *mockStore) WriteMetadata(_ context.Context, meta scraper.BookMeta) error {
 	s.mu.Lock()
 	defer s.mu.Unlock()